Coal as it is recovered from the mine (termed “run-of-mine” or ROM coal) comes in a variety of sizes and shapes and contains mineral impurities from which it must be separated. Preparing the ROM coal for other uses, involving processes known as coal preparation or cleaning, aims to sort the coal according to size, and aims to separate it from its mineral content. The mineral content of coal is the noncombustible inorganic fraction, comprised of minerals that are either detrital or authigenic in origin and that are introduced into the coal in the first or second phases of coalification. Minerals can be found in the ROM coal as combinations of larger inclusions within the coal lumps and ultrafine crystals disseminated throughout the coal lumps.
As a first step in coal cleaning, the coal is crushed to reduce its size and to free it up from the larger mineral inclusions. Assisting in this process is the fact that the coal tends to break more easily than the minerals, so that the coal can be liberated from some of the surrounding minerals by size reduction techniques using crushers, rotary breakers or other similar devices. Size differences are exploited to sort the crushed coal into different categories of pellet sizes, some of which can be used immediately if the coal is of sufficient quality. In addition, the larger lumps of coal (˜10-150 mm in length) can be treated with a technology called dense-medium separation, where the organic coal is floated free of impurities by immersing the crushed material in a high-density liquid; because the coal is less dense, it floats to the surface, while the heavier mineral matter will sink to be removed as waste.
Further crushing may be necessary if the coal is more intimately associated with minerals. The smaller-sized coal fragments can then be treated with froth flotation to separate the coal from the minerals that surround it. Using this technique, fine coal fragments can be mixed with water and other additives, then exposed to streams of air bubbles. The coal is carried to the surface in the froth, where it can be skimmed off, screened and dewatered for commercial uses, while the minerals sink to the bottom. The dewatered mass of fine coal obtained through this process is termed FC, for “filter cake.” Coal particles in the filter cake are typically about the size of sand particles.
The mineral material separated from the coal during these processes is dewatered, using for example vibratory screens, and then compacted for disposal or for further mineral recovery efforts. This waste mineral material is called coal refuse, or coal processing refuse (CPR). Depending on the type and source of the coal, the ratio of CPR to filter cake can be as high as 5:1 by weight. It may contain particles that range from microns in size to millimeters in size. The CPR may be further treated to remove useful minerals from it, or it may be disposed of as a waste material.
After these water-driven separation processes, fine particles remain in the slurry, called “fines,” which can include inorganic and organic materials (coal, clay, minerals, and the like). The fines that are coal particles can be termed “coal fines.” Separating the coal fines from the suspending medium is difficult, as the fines tend to remain suspended unless energy-intensive processes are employed to recover them. In coal mining and processing, significant quantities of coal fines are created that require disposal and handling.
The amount of coal fines in the slurry stream will vary by site depending on the efficiency of the coal processing facility. Other fines in the slurry include clays and fine mineral materials. Treating the slurry to remove all suspended solids and recover clarified water is a difficult problem. Additionally, it is highly desirable to recover the high value coal fines from the slurry. Older, less efficient coal processing facilities tend to have larger quantities of valuable coal fines ending up in the slurry stream. To increase the overall efficiency of coal processing, the ability to selectively recover the valuable coal fines from the slurry is needed. Recovering the coal-rich fines from the slurry by mechanical means is difficult, expensive, and inefficient.
Coal fines can be converted into pellets to facilitate disposal, transportation and handling. Coal-fired power plants can burn coal pellets as the fuel of choice. Pelletizing the coal fines generally requires adding an adhesive binder to the slurry containing coal fines, and using high temperatures or pressures to form the dry, consolidated pellets. Such steps are typically employed to agglomerate coal because coal particles do not naturally adhere to each other unless particle size is carefully controlled and extremely high pressures are used (over 20,000 psi for bituminous coal, for example). As an alternative to high pressure, an adhesive binder such as asphalt can be applied to bind the coal particles together. The adhesive can be expensive itself, and its use requires that a system incorporate equipment specifically to prepare and meter the adhesive, adding additional expense.
Pellet manufacture presently requires both shaping and drying. Water-soluble or water-dispersible binders are difficult to dry, and the resulting pellets are difficult to dewater. Once in pellet form, the coal product is densely consolidated, so that oxygen for combustion penetrates with difficulty. In other words, the high interfacial area characteristic of fines is drastically reduced by pellet formation, and the great combustion efficiency inherent in powder burning is lost.
Currently, then, pelletization permits fines to be disposed of in a form that is useful for combustion purposes and convenient for transport and handling. However, the pellets do not burn efficiently in a combustion chamber. It is known in the art to coat wet pellets with a hydrophobic material during processing so that residual water is trapped in the interior of the pellet; when such pellets are introduced into a boiler, the interior water vaporizes rapidly so that the pellet bursts, releasing powdered coal for combustion. However, the high heat of vaporization for water lowers the overall power output of a plant using such technology. In addition, a coating step is required, adding to the expense of manufacturing. There remains a need in the art, therefore, to offer a pelletizing process that improves upon conventional processes by enhancing efficiency of combustion and ease of handling.
In addition to coal fines waste, an enormous amount of biomass waste is generated annually. Wood waste is produced by lumber mills, for example, with wasted wood accounting for about ten percent of processed lumber. Wood waste can also be found in forests as deadwood, living biomass, or residua from timber harvesting. Lignocellulosic waste is produced by agriculture (e.g., corn stalks, wheat, hays, grasses, sugar cane bagasse, soybeans) and by processing (e.g., cotton gins). Feathers remaining from poultry farming require disposal as waste. Waste from animal husbandry includes organic material such as manure, feedstock and bedding. Additional organic waste is produced by cattle, hog, chicken, turkey and fish farming. Industrial products such as carpeting and automobile tires end up as waste that must be disposed of.
It is known in the art to form combustible organic waste materials into pellets, so that such biomass can be disposed of by combustion to generate energy. U.S. Pat. No. 6,506,223 discloses technologies for forming biomass waste material into useful combustible products. It is also known to combine biomass liquefaction products with waste coal fines to create combustible pellets, as disclosed in U.S. Pat. No. 5,916,826. There remains a need in the art, however, for improved techniques of forming combustible pellets that optimize combustion efficiency. It is further desirable to provide for disposal of both coal fines waste and biomass waste in a combustible pelletized product.